Because the majority of blacks were not Communists...
One of the major factions of blacks was the Zulu tribe, led by Chief Mangosuthu Buthelezi, who was pro-Western and Christian. Buthelezi wanted a constitution that would limit the powers of the national government, leaving much power to the individual states. Mandela (a member of the Xhosa) tribe) was the leader of the ANC and a Communist. His second in command, Joe Slovo, was a Communist from Lithuania, no doubt taking orders from Moscow.
Yes, I had mentioned Buthelezi.